Oxide material, method for preparing oxide thin film and element using said material

ABSTRACT

An oxide material characterized by that it has a perovskite structure comprising an oxide represented by ABO 3 , (Bi 2 O 2 ) 2+  (A m−1 B m O 3m+1 ) 2−  wherein A represents one kind or two or more kinds of ions selected from the group consisting of Li + , Na + , K + , Pb 2+ , Ca 2+ , Sr 2+ , Ba 2+ , Bi 3+ , Y 3+ , Mn 3+  and La 3+ , B represents one kind or two or more kinds of ions selected from the group consisting of Ru 3+ , Fe 3+ , Ti 4+ , Zr 4+ , Cu 4+ , Nb 5+ , Ta 5+ , V 5+ , W 6+  and Mo 6+ , and m represents a natural number of 1 or more, LnBa 2 Cu 3 O 7 , Z 2 Ba 2 Ca n−1 Cu n O 2n+4  or ZBa 2 Ca n−1 Cu n O 2n+3 , wherein Ln represents one kind or two or more kinds of ions selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, Z represents one kind or two or more kinds of ions selected from the group consisting of Bi, Tl and Hg, and n represents a natural number of from 1 to 5; and a catalytic substance containing one or more kinds of elements selected from the group consisting of Si, Ge and Sn.

TECHNICAL FIELD

The present invention relates to an oxide material, a process forproducing an oxide thin film, and an element using the material, andmore specifically, it relates to an oxide material that can bepreferably used for a ferroelectric memory, which is a nonvolatilememory, a process for producing an oxide thin film, and an element usingthe material.

BACKGROUND ART

A ferroelectric memory is receiving attention associated with progressof portable terminals in recent years, such as a portable telephone, anotebook computer, a personal digital assistant (PDA) and the like. Thisis because a ferroelectric memory is advantageous particularly for amultimedia equipment owing to a high writing speed and realization of alarge capacity, and no electric power is necessary for maintaining datato realize low electric power consumption.

A ferroelectric memory utilizes polarization characteristics of aferroelectric material. The direction of polarization is arbitrarilycontrolled with an external electric field to maintain binary data,i.e., “0” and “1”, and the data can be maintained even upon turning offpower.

However, only products having a small capacity of from 4 to 256 kbit arecommercialized. It is the current situation that application of aferroelectric memory to a large capacity product in Mbit level ishindered by the problem of the ferroelectric material itself.

The lamellar structure ferroelectric material (BiA_(m−1)B_(m)O_(3m+3))such as plumbum zirconate titanate (PZT; PbZr_(x)Ti_(1−x)O₃) perovskiteferroelectric material (ABO₃), strontium bismuth tantalate (SBT;SrBi₂Ta₂O₉) and bismuth titanate (BIT; Bi₄Ti₃O₁₂) highlighted underdonation of La, which are currently used, are necessarily baked at ahigh temperature of about from 600 to 800° C. for a long period of time(T. Nakamura, Technical Research Report of Institute of Electronics,Information and Communication Engineers, ED97-208 (1998), p. 25–32; T.Eshita, et al., Technical Research Report of Institute of Electronics,Information and Communication Engineers, ED98-242 (1999), p. 21–26; andM. Yamaguchi, “Studies on Formation and Evaluation of Bismuth TitanateThin Film on Silicon Substrate”, Academic Dissertation of ShibauraInstitute of Technology (1998), p. 39–47). Such crystallization at ahigh temperature for a long period of time is necessary not only forderives sufficient characteristics of the ferroelectric material itself,but also for compensating, as much as possible, deterioration inferroelectric characteristics during the production process in the casewhere the ferroelectric material is used as an element, for example,SiO₂ passivation, capacitor processing and the like, which will bedescribed later.

Therefore, in order to form a ferroelectric memory by combining aferroelectric capacitor using the ferroelectric material with asemiconductor element, such an artifice or the like is necessary thatthe ferroelectric capacitor and the transistor are formed separately dueto the high crystallization temperature for forming the ferroelectricmaterial. Accordingly, high integration of a ferroelectric memory isdifficult because of the complication of the production process,restriction in electrode material used in combination, and the like.

In general, a ferroelectric material thin film is used in aferroelectric memory, and the formation thereof is attained by using asol-gel process owing to the simplicity and excellent mass-productivityexcept for step covering property.

In the sol-gel process, a sol-gel raw material solution having a highlyvolatile component added in an excess amount of about 10% is used inorder to improve the crystallinity and to suppress at minimum deviationof the film composition after the film formation.

However, the excess addition of lead and bismuth components sometimesbrings about scattering in film composition distribution caused in theferroelectric thin film finally formed. Furthermore, the compositionaldeviation in the film accelerates formation of a hetero-phase (such asBIT and a pyrochlore phase and a fluorite phase of SBT, and the like),and makes difficult to obtain the objective ferroelectric single layer.

In the production of a ferroelectric memory, an electrode material thathas sufficient durability against the high temperature baking stepbecause of the high backing temperature for the crystallization of theferroelectric material described in the foregoing.

For example, it has been said that PZT, which is a solid solution of aPbZrO₃ antiferroelectric material and a PbTiO₃ ferroelectric material,provides a small load on an electrode material. However, it is said thata baking temperature of from 600 to 750° C. is necessary to ensure aresidual polarization value that is practically necessary (T. Nakamura,Technical Research Report of Institute of Electronics, Information andCommunication Engineers, ED97-208 (1998), p. 25–32), and thus the loadon the electrode material or the like is not small. That is, in the casewhere a PZT thin film is formed on a standard Pt electrode, so-calledfilm fatigue, i.e., precipitous deterioration of the polarization value,occurs due to repeated inversion.

Therefore, it is necessary to use a complicated complex electrode withan oxide series electrode that is excellent in controllability offatigue of an expensive ferroelectric material, such as Ir, IrO₂ and thelike, which is difficult to be processed, or an oxide electrode, such asPt/IrO₂ and the like.

On the other hand, SBT (SrBi₂Ta₂O₉: m=2) which is a bismuth lamellarstructure ferroelectric material is receiving attention as a materialthat is free of fatigue upon inversion repeated 10¹² times on a Ptelectrode and is being earnestly studied for practical application.

However, upon forming SBT into a thin film, coarse grains are aggregatedat a low density to obtain only a deteriorated surface morphology (K.Aizawa, et al., Jpn. J. Appl. Phys., vol. 39, p. 1191–1193 (2000)), andhigh integration (thin film formation) has not yet been realized at thecurrent situation.

SBT is considerably good in P-E hysteresis form but has a low residualpolarization value of from 7 to 10 μC/cm², and thus, when it is tried tobe used in a memory of reading a capacitance of a ferroelectriccapacitor having been currently commercialized, there is no margin forpolarization characteristics, and sufficient characteristics forpractical application have not yet been obtained.

Furthermore, SBT is difficult to lower the temperature forcrystallization. Specifically, in order to form SBT into a thin film,such methods have been attempted as high temperature baking at 800° C.,a long time baking of as much as 5 hours at a relatively low temperatureof about 650° C. (Y. Sawada, et la., Technical Research Report ofInstitute of Electronics, Information and Communication Engineers,ED98-240 (1999), p. 9–14), and two-step baking combining them (S.Hayashi, et al., Technical Research Report of Institute of Electronics,Information and Communication Engineers, ED98-241 (1999), p. 15–19).However, the load on the electrode material due to thermal history islarge beyond comparison to PZT, and there is a considerably largeproblem upon application of the material to practical use.

In recent years, such a method is proposed that the crystallization(baking) temperature is lowered by doping with La, and BIT (Bi₄Ti₃O₁₂:m=3) is receiving attention as a material used for the method. Thematerial contains a bismuth lamellar structure, has good fatiguecharacteristics, has a high transition temperature (Tc) of 675° C., andexhibits considerably stable material characteristics at an ordinarytemperature, as similar to SBT.

However, the material requires thermal history at 650° C. for 1 hour.Therefore, the load on the electrode material is still large (B. H.Park, B. S. Kang, S. D. Bu, T. W. Noh, J. Lee and W. Jo, Nature, vol.401, p. 682 (1999)).

The largest problem of the ferroelectric material is that giantparticles are liable to be formed (T. Nakamura, “Studies onFerroelectric Memory having Floating Gate Structure”, AcademicDissertation of Kyoto University (1998), p. 118–140) and it issignificantly difficult to form into a thin film, as similar to SBT.

In order to attain high integration and low voltage driving of aferroelectric thin film element, it is necessary to form theferroelectric material itself into an extremely thin film.

However, a thin film of 100 nm or less cannot be formed with goodreproducibility due to the surface c of the ferroelectric material.Further, even though it can be formed to have a thickness of 100 nm orless, the ferroelectric characteristics are quickly deteriorated (K.Aoki, et al., Technical Research Report of Institute of Electronics,Information and Communication Engineers, ED98-245 (1999), p. 43–49).

It is considered that the surface morphology deterioration of theferroelectric thin film is caused because the crystallization occursfrom a lower electrode (for example, a platinum electrode), i.e., fromthe lowermost surface of the ferroelectric thin film, irrespective tothe film forming method, such as the sol-gel process and the MOCVDprocess, so as to provide a form having grains convex upward aggregated.Furthermore, although the compatibility between the lower electrodematerial and the ferroelectric material is poor, the crystallization ofthe ferroelectric material depends only on the catalytic feature of theplatinum electrode, and therefore, the density of initialcrystallization nucleus formation of the ferroelectric material is low.Therefore, when the ferroelectric material is formed into a thin film of100 nm or less, the ferroelectric thin film grows in an island form butnot covering the entire lower electrode. As a result, considerablycoarse surface morphology is obtained, and the resulting ferroelectricthin film increases the leakage electric current density. It has beenknown that a ferroelectric thin film derived from an organic metallicmaterial as a starting material has a large amount of carbon remaining,and it is also one factor of increase in leakage electric currentdensity.

Moreover, the ferroelectric material is deteriorated in ferroelectriccharacteristics in a reducing atmosphere (Y. Shimamoto, et al., Appl.Phys. Lett., vol. 70, p. 1–2 (1997)).

For example, in the case where the ferroelectric material is used as acapacitor, SiO₂ passivation using ozone TEOS or the like is generallycarried out as a protective film of the ferroelectric capacitor. At thistime, the ferroelectric capacitor having a platinum upper electrodeformed is exposed to a hydrogen atmosphere. Therefore, hydrogenactivated by the catalytic function of the platinum upper electrodereduces the ferroelectric material to cause structural breakage of theferroelectric material, and as a result, the ferroelectriccharacteristics are greatly deteriorated.

It is considered that the reasons therefor are that the ferroelectricmaterial is a material having a strong ionic bonding property (while itis said that an ionic bond is a strong bond, it is considerably weakagainst attack by ions), and the effective area receiving attack by thedeteriorated surface morphology is large.

Accordingly, in order to restore the characteristics of theferroelectric material suffering the structural breakage, re-oxidationof the ferroelectric material is carried out, for example, it is againbaked in an oxygen atmosphere, after the SiO₂ passivation.

However, the oxidation applies unnecessary thermal history to theelement, and moreover, the ferroelectric characteristics oncedeteriorated cannot be completely restored by the re-oxidation.

Under the current circumstances, accordingly, there are variousproblems, such as (1) the excess addition of a highly volatilecomponent, such as lead, bismuth and the like, (2) the difficulty inobtaining a ferroelectric single phase, (3) the high crystallizingtemperature, (4) the presence of a large amount of carbon residue in thefilm, (5) the difficulty in forming a thin film of 100 nm, (6) thedecomposition under a reducing atmosphere, such as hydrogen or the like,and the like problems, although various kinds of ferroelectric materialshave been studies, and thus high integration of ferroelectric thin filmelements has not yet been realized.

PZT, a representative perovskite ferroelectric material (ABO₃), causespolarization inversion fatigue on a conventional platinum electrode, andSBT and BIT, a bismuth lamellar structure ferroelectric material(BiA_(m−1)B_(m)O_(3m+3)), are difficult to be formed into a thin filmdue to the deteriorated surface morphology. That is, the ferroelectricmaterials that are expected to be applied to a memory element undercurrent situation involve respective problems.

The problems are the case not only in the ferroelectric material, butalso in a SrRuO₃ perovskite electrode material and a perovskite oxidematerial, such as (Ba, Sr)TiO₃, SiTiO₃ and the like, which is expectedas a high dielectric constant gate oxide film for a next-generationDRAM.

DISCLOSURE OF THE INVENTION

The invention provides an oxide material which is a solid solution of aperovskite or perovskite lamellar structure oxide having dissolvedtherein a catalytic substance containing one or more kinds of IV groupelements selected from the group consisting of Si, Ge and Sn.

Also, the invention provides a method for produing an oxide materialwhich comprises: mixing a gel solution for forming a perovskite orlamellar perovskite structure oxide formed by mixing alkoxides ororganic acid salts of two or more kinds of metals selected from thegroup consisting of Li, Na, K, Pb, Ca, Sr, Ba, Bi, La, Ru, Fe, Ti, Zr,Cu, Nb, Ta, V, W and Mo, with a gel solution for forming a catalyticsubstance formed by mixing alkoxides or organic acide salts of one ormore kinds of metals selected from the group consisting of Ca, Ba, Pb,Zn, Sr, Mg, Fe, Fe, B, Al, In, Y, Sc, Sb, Cr, Bi, Ga, Cu, Mn, Zr, Ti,Mo, W, V, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu,and one or more kinds of IV group metals selected from the groupconsisting of Si, Ge and Sn, in an anhydrous state; and coating andcalcinating the resulting mixture on a substrate.

Further, according to the invention, it provides a mixed anhydroussolution for forming a complex oxide material comprising apolycondensation product for forming a perovskite or perovskite lamellarstructure oxide material represented by ABO₃,(Bi₂O₂)²⁺(A_(m−1)B_(m)O_(3m+1))²⁻, wherein A represents one kind or twoor more kinds of ions selected from the group consisting of Li⁺, Na⁺,K⁺, Pb²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Bi³⁺, Y³⁺, Mn³⁺ and La³⁺, B represents onekind or two or more kinds of ions selected from the group consisting ofRu³⁺, Fe³⁺, Ti⁴⁺, Zr⁴⁺, Cu⁴⁺, Nb⁵⁺, Ta⁵⁺, V⁵⁺, W⁶⁺ and Mo⁶⁺, and mrepresents a natural number of 1 or more, LnBa₂Cu₃O₇,Z₂Ba₂Ca_(n−1)Cu_(n)O_(2n+4) or ZBa₂Ca_(n−1)Cu_(n)O_(2n+3), wherein Lnrepresents one kind or two or more kinds of ions selected from the groupconsisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yband Lu, Z represents one kind or two or more kinds of ions selected fromthe group consisting of Bi, Tl and Hg, and n represents a natural numberof from 1 to 5,

one or more kinds of oxides selected from the group consisting of CaO,BaO, PbO, ZnO, SrO, MgO, FeO, Fe₂O₃, B₂O₃, Al₂O₃, In₂O₃, Y₂O₃, Sc₂O₃,Sb₂O₃, Cr₂O₃, Bi₂O₃, Ga₂O₃, CuO₂, MnO₂, ZrO₂, TiO₂, MoO₃, WO₃, V₂O₅ anda lantanoid oxide; and one or more kinds of IV group metallic oxidesselected from the group consisting of SiO₂, GeO₂ and SnO₂.

Also, the invention provides a substrate on which an electroconductivematerial film and the preceding oxide material thereon are formed, andan element in which an upper electrode is formed on the precedingsubstrate, or a semiconductor element in which the preceding oxidematerial film and an electroconductive material film are formed on asemiconductor substrate and further a pair of impurity diffusion layersare located on both sides of the preceding electroconductive materialfilm and on the preceding semiconductor substrate surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the latitudecoefficient and the number of atoms.

FIG. 2 is a graph showing the results of differential thermal analysis(TG-DTA) of a BSO solution, a BIT solution and a BSO—BIT mixed solution.

FIG. 3 is a diagram showing the EDX patterns of the BIT—BSO mixedsolution and the BIT solution.

FIG. 4 is a principal diagram showing the state of a raw materialsolution formed by mixing a BSO sol-gel solution and a PZT sol-gelsolution.

FIG. 5 is a diagram showing (a) the XRD pattern of the oxide materialfilm after calcination in the case where the film is formed by using aBIT—BSO mixed solution, (b) the XRD pattern of the oxide material filmafter crystallization, and (c) the XRD pattern of the oxide materialfilm after calcination in the case where the film is formed byalternately coating a BSO solution and a BIT solution.

FIG. 6 is (a) a diagram showing the state in the case where the BIT—BSOmixed solution is coated on a platinum electrode, and (b) a diagramshowing the state in the case where the BSO solution and the BITsolution are alternately coated.

FIG. 7 is (a) a diagram showing the hysteresis characteristics of theoxide material film obtained in FIGS. 6( a), and (b) a diagram showingthe hysteresis characteristics of the oxide material film obtained inFIG. 6( b).

FIG. 8 is a diagram for describing the state in the crystallizationprocess of the oxide material film.

FIG. 9 is a flowchart for preparing a PZT sol-gel solution.

FIG. 10 is a flow chart for preparing a BSO sol-gel solution.

FIG. 11 is a flow chart for preparing a SBT sol-gel solution.

FIG. 12 is a diagram showing the XRD pattern of an oxide material filmin the case where crystallization is carried out by using a raw materialsolution having a water content introduced therein.

FIG. 13 is a graph showing the hysteresis characteristics of the oxidematerial film of FIG. 12.

FIG. 14 is the XRD patterns of BIT and BSO—BIT (R=0.4) according to theinvention.

FIG. 15 is a diagram showing the crystallization process of Bi₄Ti₃O₁₂and Bi₂SiO₅-added Bi₄Ti₃O₁₂ (R=0.4) according to the invention.

FIG. 16 is the XRD patterns of SrBi₂Ta₂O₉ and Bi₂SiO₅-added SBT (R=0.33)thin films and PbZr_(0.52)Ti_(0.48)O₃ and Bi₂SiO₅-addedPbZr_(0.52)Ti_(0.48)O₃ (R=0.1) thin films.

FIG. 17 is the D-E hysteresis characteristics of a Bi₂SiO₅-addedBi₄Ti₃O₁₂ (R=0.4), Bi₂SiO₅-added SrBi₂Ta₂O₉ (R=0.33) and Bi₂SiO₅-addedPbZr_(0.52)Ti_(0.48)O₃ (R=0.1) thin films.

FIG. 18 is a graph showing the film fatigue characteristics of aBi₂SiO₅-added Bi₄Ti₃O₁₂ (R=0.4), Bi₂SiO₅-added SrBi₂Ta₂O₉ . (R=0.33) andBi₂SiO₅-added PbZr_(0.52)Ti_(0.48)O₃ (R=0.1) thin films.

FIG. 19 is a diagram showing the crystalline structure of BIT.

FIG. 20 is a diagram showing the crystalline structure of BSO.

FIG. 21 is a diagram showing the superlattice structure of BSO and BIT.

FIG. 22 is a diagram showing the reduction resistance upon annealing aPt/BIT/Pt capacitor according to the invention in N₂ containing 3% of H₂at 400° C.

FIG. 23 is a diagram comparing the XRD patterns of a BIT+Bi₄Si₃O₁₂ andBIT+SBO thin films according to the invention and a BIT thin film.

FIG. 24 is a diagram showing the D-E hysteresis characteristics of aBi₄Ge₃O₁₂—Bi₄Ti₃O₁₂ ferroelectric thin film having a thickness of 100 nmaccording to the invention.

FIG. 25 is a diagram showing the film fatigue characteristics after 10¹⁰times polarization inversions of the Bi₄Ge₃O₁₂—Bi₄Ti₃O₁₂ ferroelectricthin film having a thickness of 100 nm according to the invention.

FIG. 26 is a diagram showing the P-E hysteresis characteristics ofBSO-added BIT and BSO-added SBT having a film thickness of from 25 to100 nm according to the invention, and showing an applied electric fieldat an axis of abscissa.

FIG. 27 is a diagram showing the P-E hysteresis characteristics ofLSO-added BIT and LSO-added SBT having a film thickness of from 25 to100 nm according to the invention, and showing an applied voltage at anaxis of abscissa.

FIG. 28 is a diagram showing the P-E hysteresis characteristics ofBSO-added BLT having a thickness of 10 nm according to the invention,and showing an applied voltage at an axis of abscissa.

FIG. 29 is a diagram showing the XRD patterns of a BIT film of R=0.4(BSO=0.2), which is an LSO-added BIT thin film.

FIG. 30 is a diagram showing change of hysteresis characteristics of anLSO-added BIT thin film according to the invention upon changing R=0.2(BSO=0), R=0.4 (BSO=0.2), R=0.8 (BSO=0.6), R=1.6 (BSO=1.4) and R=3.2(BSO=3.0).

FIG. 31 is a diagram showing the XRD patterns of a PZT film of R=0.025to 0.2 in a BSO-added PZT thin film according to the invention.

FIG. 32 is a diagram showing the hysteresis characteristics of the thinfilm shown in FIG. 31.

FIG. 33 is a diagram showing the ferroelectric characteristics of solidsolution thin films of various kinds of catalytic substances with BIT.

FIG. 34 is a diagram showing the ferroelectric characteristics of solidsolution thin films of various kinds of catalytic substances with SBT.

FIG. 35 is a diagram showing the ferroelectric characteristics of solidsolution thin films of various kinds of catalytic substances with PZT.

FIG. 36 is a diagram showing the electric characteristics of a BSO—BST(R=0.2) thin film.

FIG. 37 is a diagram showing the XRD pattern of a BSO—Bi₂Sr₂Ca₂Cu₃O_(x)(Bi2223) crystal.

FIG. 38 is a schematic cross sectional view showing an element using theoxide material according to the invention.

FIG. 39 is a diagram showing the XRD pattern of a BIT—BSO crystal.

FIG. 40 is a diagram showing the diode characteristics of an elementusing the oxide material thin film of FIG. 39.

FIG. 41 is a diagram showing the XRD pattern of an SBT—BSO crystal.

FIGS. 42 and 43 are diagrams showing the transistor characteristics of atransistor using the oxide material film of FIG. 41.

BEST MODE FOR CARRYING OUT THE INVENTION

The oxide material of the invention contains a solid solution of aperovskite or perovskite lamellar structure oxide having dissolvedtherein a catalytic substance containing at least one kind of a IV Groupelement selected from the group consisting of Si, Ge and Sn. Accordingto the constitution, the characteristics inherent to the perovskite orperovskite lamellar structure oxide can be maintained or improved, andat the same time, in particular, crystallization at a low temperaturecan be realized.

In the invention, the perovskite or perovskite lamellar structure oxideincludes compounds having various characteristics, such as ferroelectricmaterials, superconductive oxides and the like, which are mainlyascribed to the crystalline structure thereof. Examples thereof includecompounds represented by general formulae, such as ABO₃,(Bi₂O₂)²⁺(A_(m−1)B_(m)O_(3m+1))²⁻, LnBa₂Cu₃O₇,Z₂Ba₂Ca_(n−1)Cu_(n)O_(2n+4), ZBa₂Ca_(n−1)Cu_(n)O_(2n+3) and the like. Inthe formulae, A represents one kind or two or more kinds of ionsselected from the group consisting of Li⁺, Na⁺, K⁺, Pb²⁺, Ca²⁺, Sr²⁺,Ba²⁺, Bi³⁺, Y³⁺, Mn³⁺ and La³⁺, B represents one kind or two or morekinds of ions selected from the group consisting of Ru³⁺, Fe³⁺, Ti⁴⁺,Zr⁴⁺, Cu⁴⁺, Nb⁵⁺, Ta⁵⁺, V⁵⁺, W⁶⁺ and Mo⁶⁺, Ln represents one kind or twoor more kinds of ions selected from the group consisting of Y, La, Ce,Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, Z represents onekind or two or more kinds of ions selected from the group consisting ofBi, Tl and Hg, m represents a natural number of 1 or more, and nrepresents a natural number of from 1 to 5.

The catalytic substance in the invention is constituted by containing atleast one kind of a IV Group element selected from the group consistingof Si, Ge and Sn. Upon heating in the crystallization process of theoxide material, the catalytic substance exerts catalytic functions, suchas increasing the reaction rate for forming the perovskite or lamellarperovskite structure oxide, accelerating formation of crystallinenuclei, and decreasing the crystallization energy, and is stablypresent. Upon forming the crystalline nuclei on the surface thereof, thecatalytic functions thereof are terminated, and thereafter, upondecreasing the temperature, it forms a coherent solid solution with asilicate or a germanate and the perovskite or lamellar perovskitestructure oxide. In other words, it exerts two kinds of functions, i.e.,the catalytic function for forming the perovskite or lamellar perovskitestructure oxide at a low temperature and a material for forming theoxide.

Specifically, the catalytic substance suitably has a function ofaccelerating crystallization at a temperature lower than the perovskiteor lamellar perovskite structure oxide constituting the solid solution,and it also suitably has good lattice compliance with the perovskite orlamellar perovskite structure oxide and a lamellar structure. In thecase where it has a lamellar structure, in particular, it is receivingattention as a catalytic substance in recent years owing to a largesurface area thereof (A. Ozaki, Catalytic Functions “Chapter 4,Production of Catalysts, 4.1 Production of Porous Materials” (1986), p.74–78), which can be preferably used.

Specifically, examples thereof include a complex oxide containing atleast one kind of oxide selected from the group consisting of CaO, BaO,PbO, ZnO, SrO, MgO, FeO, Fe₂O₃, B₂O₃, Al₂O₃, In₂O₃, Y₂O₃, Sc₂O₃, Sb₂O₃,Cr₂O₃, Bi₂O₃, Ga₂O₃, CuO₂, MnO₂, ZrO₂, TiO₂, MoO₃, WO₃, V₂O₅, alantanoid oxide and the like, and at least one kind of a IV Groupmetallic oxide selected from the group consisting of SiO₂, GeO₂, SnO₂and the like.

Specific examples of the complex oxide include materials represented byX₂SiO₅, X₄Si₃O₁₂, X₂GeO₅, X₄Ge₃O₁₂, X₂SnO₅and X₄Sn₃O₁₂ (wherein Xrepresents Ca²⁺, Ba²⁺, Pb²⁺, Zn²⁺, Sr²⁺, Mg²⁺, Fe²⁺, Fe³⁺, B³⁺, Al³⁺,In³⁺, Y³⁺, Sc³⁺, Sb³⁺, Cr³⁺, Bi³⁺, Ga³⁺, Cu⁴⁺, Mn⁴⁺, Zr⁴⁺, Ti⁴⁺, Mo⁶⁺,V⁵⁺, La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺,Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺ and the like), and the like.

More specifically, examples thereof include Bi₂SiO₅, La₂SiO₅, Y₂SiO₅,Bi₄Si₃O₁₂, La₄Si₃O₁₂, Y₄Si₃O₁₂, Bi₂GeO₅, La₂GeO₅, Y₂GeO₅, Bi₄Ge₃O₁₂,La₄Ge₃O₁₂, Y₄Ge₃O₁₂, Bi₂SnO₅, La₂SnO₅, Y₂SnO₅, Bi₄Sn₃O₁₂, La₄Sn₃O₁₂,Y₄Sn₃O₁₂, mixtures of two or more kinds thereof, such asBi_(2-x)La_(x)SiO₅ and La_(4-x)Y_(x)Si₃O₁₂, and the like.

For example, a 1/1 complex oxide of bismuth oxide and silicon oxide isBi₂SiO₅, and Bi₂SiO₅ is such an oxide material that is crystallized at alow temperature of about 400° C. (M. Yamaguchi, et al., Proceedings ofthe 2000 12th IEEE International Symposium on Applications ofFerroelectrics, vol. 2, p. 629–632 (2001)). Bi₂SiO₅has good latticecompliance with the most of the oxide described later, and therefore, anoxide can be selectively crystallized at a low temperature. Bi₂SiO₅ isalso significantly stable owing to four-coordination SiO₂ thereof.

The oxide material of the invention is a solid solution, and in otherwords, it has such a structure that has a phase formed by randomlyreplacing atoms at lattice points with atoms of a different kind, orintroducing atoms of a different kind into lattice spaces to bestatistically distributed, i.e., a mixed phase formed by dissolving asubstance of a different kind into a crystalline phase. The lamellarperovskite structure oxide has such a crystalline structure formed byintroducing atoms of a different kind into a crystalline phase, but ithas been known that it is not a solid solution (Carlos A. Pazde Araujo,“Advanced Process for Ferroelectric Memory” Science Forum, p. 35–43(1999)). Therefore, examples of the oxide material of the inventioninclude an oxide material having a form of a solid solution of acatalytic substance containing a IV Group element and a perovskite orlamellar perovskite structure oxide material, and containing Si⁴⁺, Ge⁴⁺or Sn⁴⁺ at positions of cations in the crystalline lattice constitutingthe perovskite or lamellar perovskite structure oxide, as well as amaterial containing Si⁴⁺, Ge⁴⁺ or Sn⁴⁺ at centers of oxygen octahedronsconstituting the perovskite or lamellar perovskite structure oxide. Theoxide material of the invention may not be in such a state that thecatalytic substance containing a IV Group element and the perovskite orlamellar perovskite structure oxide completely form a solid solution,but may be such a material that contains Si⁴⁺, Ge⁴⁺ or Sn⁴⁺ at thepositions of cations in the crystalline lattice or at the centers ofoxygen octahedrons.

In the case where the cations in the crystalline lattice or the ions atthe centers of the oxygen octahedrons constituting the perovskite orlamellar perovskite structure oxide are replaced with Si⁴⁺, Ge⁴⁺ orSn⁴⁺, the extent of the replacement is not particularly limited. Forexample, in the case of Bi₄Ti₃O₁₂, a difference, such as decrease incrystallization temperature, appears in Bi₄Ti_(2.9)Si_(0.1)O₁₂, in whichTi is replaced with 0.1 of Si, and improvement in electriccharacteristics appears in Bi₄Ti_(2.5)Si_(0.5)O₁₂. Therefore, 0.1 ormore, and more particularly 0.5 or more atom per one molecule can beexemplified. From another standpoint, the solid solution preferablycontains the perovskite or lamellar perovskite structure oxide havingthe IV Group element dissolved therein at a ratio of about from 1/0.01to 1/5, preferably about from 1/0.01 to 1/0.8, and more preferably aboutfrom 1/0.01 to 1/0.5. From still another standpoint, the cations in thecrystalline lattice or the ions at the center of the oxygen octahedronsconstituting the perovskite or lamellar perovskite structure oxide maybe replaced with about 50% or less, preferably about 40% or less, andmore preferably about 35% or less, of Si⁴⁺, Ge⁴⁺ or Sn⁴⁺.

In the case of the ABO₃ perovskite oxide material, in general, there arerestrictions in ionic radius of the crystalline lattice, for example,the respective constitutional ions of the oxygen octahedrons, as anexistence condition as a crystal. This is expressed by a latitudecoefficient t, and the latitude coefficient t=(R_(A)+R₀)/(2(R_(B)+R₀) isnecessarily in a range of 0.8<t<1.02.

In the case of (Bi₂O₂)²⁺(A_(m−1)B_(m)O_(3m+1))²⁻, bismuth lamellarperovskite material, the restriction in latitude coefficient t withrespect to the ions constituting the virtual perovskite lattice is0.81<t<0.93 for m=2 (0.83<t<0.91 for m=3), 0.85<t<0.89 for m=4, and0.86<t<0.87 for m=5, and it has been known that when m is increased, thelatitude range of t is narrowed (T. Takenaka, Report on Workshop forApplied Electronic Properties, Japan Society of Applied Physics,Workshop for Applied Electronic Properties vol. 456, p. 1–8 (1994)).Since the latitude coefficient t for m=3 is not disclosed, it can beobtained by extrapolating the proportional relationship shown in FIG. 1from the minimum and maximum latitude coefficients for m=2, 4, and 5.

Upon applying the latitude coefficient t to PZT, SBT and BIT, in thecase where the lattice, for example, the centers of the oxygenoctahedrons are replaced with Si⁴⁺, the latitude coefficient t becomest=1.10 (Å) for PZT, t=1.09 (Å) for SBT, and t=1.04 (Å) for BIT, all ofwhich are not applied to the latitude range. The ionic radius isdesignated as an average value 0.84×0.52+0.75×0.48=1.18 (Å) in the caseof a solid solution, such as PZT, as Zr/Ti=0.52/0.48, by using 1.26 (Å)for O²⁻, 1.33 (Å) for Pb²⁺, 1.32 (Å) for Sr²⁺, 1.17 (Å) for Bi³⁺, 1.17(Å) for La³⁺, 0.75 (Å) for Ti⁴⁺, 0.84 (Å) for Zr⁴⁺, 0.78 (Å) for Ta⁵⁺,and 0.40 (Å) for Si⁴⁺, and the empirical ionic radii by Shannon (1976)and Prewitt (1969 and 1970).

According thereto, the replacement of the crystalline lattice of theperovskite structure or lamellar perovskite structure oxide material,for example, the centers (B sites) of the oxygen octahedrons, with Si⁴⁺provides a latitude coefficient t outside the latitude range, and thusit is understood that replacement of the B sites with Si⁴⁺ isconsiderably difficult. This is also apparent from U.S. Pat. No.5,519,234, which discloses that a metal, such as Ti, Ta, Hf, W, Nb orZr, can be replaced as a B site ion in the SBT, but does not disclosethat it can be replaced with Si⁴⁺ or the like, which is a metalloid.

In order to replace at the lattice points of the perovskite or lamellarperovskite structure, SiO₆ ⁸⁻, which is a six-coordinate octahedralstructure, is necessary, but a IV Group element has a coordinationnumber of 4, and has, for example, SiO₂ (SiO₄ ⁴⁻), which is atetrahedral structure. Furthermore, a large amount of perovskitestructure materials, such as CaSiO₃ and MgSiO, are present under a highpressure of 20 GPa (200,000 atm) or more in the mantle of the earth at600 km below ground (S. K. Saxena, et al., Science, “Stability ofPerovskite (MgSiO₃) in the Earth's Mantle”, vol. 274, p. 1357–1359(1996)), and it has been known that the six-coordinate structure of theIV Group element (Si, Ge and Sn) is present only under such a highpressure environment.

In spite of the foregoing, the strongest covalency can be introducedinto the crystalline structure according to the foregoing constitutionin the invention, i.e., the atoms at the lattice points of thecrystalline phase can be randomly replaced with a different kind ofatoms, the IV Group element (Si, Ge and Sn) other than C and Pb (C has atoo small ionic radius, and Pb has a too large ionic radius), or the IVGroup element can be statistically distributed into lattice spaces, andthe reactivity of an ionic bond, which is significantly reactive to aprocess concerning ions, such as active hydrogen (H⁺) and an appliedelectric field (e⁻), can be prevented in the perovskite structure oxidematerial. As a result, breakage of the perovskite structure can beprevented under a hydrogen reducing atmosphere.

The perovskite or lamellar perovskite structure oxide requires largethermal energy (baking temperature) for forming crystalline nucleithereof, but only low thermal energy is required for growing (T. Kijima,S. Satoh, H. Matsunaga and M. Koba, “Ultra-Thin Fatigue-Free Bi₄Ti₃O₁₂Films for Ferroelectric Memories”, Jpn. J. Appl. Phys., vol. 35, p.1246–1250 (1996)). Therefore, the reaction from a raw materialcontaining elements constituting the oxide material to the oxidematerial is accelerated, and the activation energy upon formingcrystalline nuclei of the oxide material is lowered, by using thecatalytic substance, and as a result, the crystallization temperaturecan be lowered.

Specifically, the temperature upon baking for crystallizing the solidsolution can be appropriately adjusted by the characteristics of theoxide material to be finally obtained, the compositions of thesolutions, or the like, and in general, in the case where a thin film isformed by using a gel solution for forming the perovskite or lamellarperovskite structure oxide, such a temperature can be exemplified asabout from 450 to 600° C. for PbZr_(0.52)Ti_(0.48)O₃ (PZT), about from500 to 650° C. for Bi₄Ti₃O₁₂ (BIT) and Bi_(4−x)La_(x)Ti₃O₁₂ (BLT), andabout from 500 to 650° C. for SrBi₂Ta₂O₉ (SBT).

Furthermore, decomposition of the raw material constituting theperovskite or lamellar perovskite structure oxide can be accelerated byusing the IV Group element-containing catalytic substance, and thus C inthe organic metallic raw materials can be removed.

The oxide material of the invention is a solid solution of the IV Groupelement-containing catalytic substance and the perovskite or lamellarperovskite structure oxide, and they are preferably present as a mixturein the oxide material. In general, the perovskite or lamellar perovskitestructure oxide causes a low forming density of crystalline nucleibecause it is crystallized by using only the catalytic function of theelectroconductive film (Pt metal is generally used) used as the lowerelectrode. Therefore, at the time when the crystalline nuclei continueto grow to form a film in the final phase, coarse surface morphology,which is convex upward, is obtained, and the applied electric fieldbecomes a leakage electric current with a thickness of 100 nm or less,whereby the characteristics of the oxide material cannot be effectivelyutilized. However, in the case where they are present as a mixture,crystalline nuclei can be formed all over the film, and accordingly, theoxide material can be formed into a dense thin film.

The perovskite or lamellar perovskite structure oxide (such as BIT orthe like) is selectively formed on the surface of the IV Groupelement-containing catalytic substance. As a result, they can completelyform a solid solution to obtain a perovskite single phase. There aresome cases where they do not completely form a solid solution with theIV Group element-containing catalytic substance remaining, but, even inthese cases, the resulting oxide material exhibits good characteristicsequivalent to the perovskite single phase. For example, in the casewhere the perovskite or lamellar perovskite structure oxide is aferroelectric material, the catalytic material is also polarized assimilar to the polarization of the ferroelectric by the displacement dueto the polarization of the ferroelectric material, and it functions as apassive ferroelectric material, whereby the characteristics of theferroelectric material are not deteriorated.

In the invention, the IV Group element-containing catalytic substanceaccelerates the formation of the crystalline nuclei of the perovskite orlamellar perovskite structure oxide upon heating in the crystallizationprocess, and it is stably present at this time. For example, SiO₂ (orGeO₂ or SnO₂) is four-coordination and is stable. After forming thecrystalline nuclei on the surface of the substance, a large compressionstress occurs upon decreasing the temperature owing to the difference inthermal expansion between silicate or germanate and the perovskite orlamellar perovskite structure oxide. Silicon or the like is replaced inthe crystalline lattice of the perovskite or lamellar perovskitestructure oxide by the function of the compression stress. Whiledepending on the mode of combination of the IV Group element-containingcatalytic substance and the perovskite or lamellar perovskite structureoxide, for example, a solid solution of Bi₄Ti₃O₁₂ and Y₂O₃—SiO₂ isformed to replace the so-called A site ions and B site ions of theperovskite or lamellar perovskite structure oxide at the same time, andthus, (Bi,Y)₄(Ti,Si)₃O₁₂can be finally obtained, whereby oxide materialshaving various characteristics, such as ferroelectric characteristics,can be formed.

The oxide material of the invention will be described in detail belowwith reference to the drawings.

EXAMPLE 1

Catalytic Property of Bi₂SiO₅ (BSO)

A BSO sol-gel solution was spin-coated on a platinum electrode formed ona substrate, and an organic component was removed on a hot-plate (400°C.) to produce a BSO thin film of 100 nm.

The substrate with platinum having the BSO thin film formed thereon wasimmersed in a 1% ammonia aqueous solution. At this time, Bi₃+in the BSOthin film became Bi(OH)₃ in the ammonia aqueous solution, and as aresult H⁺ became excessive in the aqueous solution to exhibit weakacidity of pH 5.

Differential thermal analysis (TG-DTA) was carried out by using a 10% byweight Bi₄Ti₃O₁₂ (BIT) solution, a 10% by weight BSO solution and aBIT—BSO mixed solution (a mixed solution obtained by mixing 0.4 mole ofBSO per 1 mole of BIT, R=0.4) respectively. The results are shown inFIG. 2.

It is understood from FIG. 2 that the BSO solution is crystallized at alow temperature of about 350° C. It is also understood that the BITsolution is difficult to be crystallized since it exhibits asignificantly broad crystallization peak in the vicinity of 600° C. TheBIT—BSO mixed solution has a peak in the vicinity of 350° C., which isconsidered to be ascribed to BSO, and subsequently, a clearcrystallization peak in the vicinity of 400° C. This is because BSO isfirstly crystallized, and then BIT is crystallized on the surface of BSOat a low temperature owing to the function of BSO crystals.

Upon comparing the TG charts in FIG. 2, the weight is graduallydecreased until 800° C. in the case of the BIT solution. On the otherhand, the weight decrease in the case of BIT—BSO mixed solution is largeuntil 400° C., but there is substantially no change thereafter. This isbecause BSO functions as an acidic catalyst in the BIT—BSO mixedsolution, which converts the organic metal to a ferroelectric materialthrough sufficient decomposition via metallic ions, and the carboncomponent to CO₂, owing to the function of BSO as an acidic catalyst.

Thin films formed by using the BIT—BSO mixed solution and the BLTsolution are subjected to EDX (energy dispersive X-ray) analysis asshown in FIG. 3, and it is found that substantially no carbon remains inthe material in the case where the BIT—BSO mixed solution is used.

It has been found from the foregoing that decomposition of the organicmetal is accelerated, and the oxide material is crystallized at a lowtemperature in the lamellar perovskite structure oxide (BIT) by usingthe IV Group element-containing catalytic substance (BSO).

EXAMPLE 2

Film Formation by Sol-Gel Process

5% by weight of a BIT sol-gel solution and 1% by weight of BSO sol-gelsolution were mixed to prepare a BIT—BSO mixed solution (R=0.4). Themixed solution in this case was in such a state that the respectivecomponents were present as a mixture, as shown in FIG. 4.

On a platinum electrode, the following operation was repeated four timesby using the mixed solution, i.e., (1) spin coating (500 rpm for 5seconds and 4,000 rpm for 20 seconds), (2) drying (in the atmosphere at50° C. for 2 minutes), and (3) calcination (in the atmosphere at 400° C.for 5 minuets), to make a thickness of 100 nm. It was confirmed that thefilm thus obtained was in the state where only BSO was crystallized asshown in FIG. 5( a), and the crystallized BSO was surrounded byamorphous BIT.

Subsequently, (4) baking (crystallization) was carried out (at 600° C.for 15 minutes, RTA in oxygen of 1 kg/cm²), and an upper platinumelectrode was formed on the resulting oxide material thin film.

Thereafter, (5) post-annealing was carried out (at 500° C. for 5minutes, RTA in oxygen of 2 kg/cm²).

As a result of XRD measurement of the oxide material film after thecrystallization, a reflection peak of the BSO crystal was completelydiminished as shown in FIG. 5( b).

The measurement of hysteresis characteristics of the resulting oxidematerial film revealed good hysteresis characteristics as shown in FIG.7( a).

For comparison, a 5% by weight BIT sol-gel solution and a 1% by weightBSO sol-gel solution were respectively prepared, film were formed in thesame manner as in the operations (1) to (3) except that 14 nm per onelayer for BIT and 7 nm per one layer for BSO were accumulated by 9layers, respectively, to make a total thickness of about 100 nm. Thetotal composition of the film was adjusted to R=0.4.

The operations (4) and (5) were carried out to attain crystallization,and thus it was confirmed that the resulting oxide material film hadcrystallized BIT layers at 500° C., which was different from theconventional BIT film. This was because the decrease in crystallizationtemperature range was ascribed to a sort of catalytic function of BSOcrystals. However, the result of XRD measurement reveals thatcrystallization peaks of BSO and BIT were mixedly present to fail toform a single layer as shown in FIG. 5( c).

The measurement of hysteresis characteristics of the resulting oxidematerial film revealed that there are no hysteresis characteristics asshown in FIG. 7( b).

The foregoing results show that the lamellar perovskite structure of BSOchanges in crystalline structure in the BIT—BSO thin film.

In other words, the foregoing results mean that the SiO₄ ⁴⁻ tetrahedronis changed in structure to the SiO₆ ⁸⁻ octahedron, and finally changedinto Bi₄(Ti,Si)₃O₁₂ containing Si as B site ions in 30% or more. In thecase where the structure is established, the coordination number of Siis changed from 4 to 6, and at the same time, the ionic radius ischanged from 0.04 nm to 0.054 nm. It is considered that this is a resultof formation of a large compression stress in the BIT—BSO thin filmbecause Si under the atmospheric environment has a too small ionicradius and does not form a perovskite structure, but a perovskitestructure oxide is present under a compression stress of 20 GPa in theunderearth mantle (Irifune, T. & Ringwood A. E., “Phase transformationin subducted oceanic crust and buoyancy relationship at depth of 600–800km in the mantle”, Earth Planet. Sci. Lett. vol. 117, p. 101–110 (1993),Surendra K. S., et al., “Stability of Perovskite (MgSiO3) in the Earth'sMantle”, Science, vol. 274, p. 1357–1359 (1996), and Dubrovinsky, L. S.,et al., “Experimental and theoretical identification of a newhigh-pressure phase of silica”, Nature, vol. 388, p. 362–365 (1997)).

The difference in thermal expansion coefficient between BSO and BITplays an important role on the formation of the large compressionstress. That is, owing to the use of the solution having BSO gel and BITgel, which are present as a mixture, the BSO crystals are surrounded bythe BIT crystals, and upon decreasing the temperature for cooling aftercrystallization annealing from a high temperature to room temperature,such a compression stress is applied that crushes (changes crystallinestructure) BSO by BIT. Under consideration that BSO contains SiO₂ as amain constitutional substance and has the similar thermal expansioncoefficient as quartz (Si), the thermal expansion coefficient of BSO isas small as it can be ignored. The linear thermal expansion coefficientof BIT is calculated (Subbarao, E. C, “Ferroelectric in Bi₄Ti₃O₁₂ andIts Solid Solutions”, Phys. Rev., vol. 122, p. 804–807 (1961)), and thecompression stress generated by compression of the BIT crystals(Ishikawa, H., Sato, T. and Sawaoka, “Epitaxial growth of strain-free Gefilms on Si substrates by solid phase epitaxy at ultrahigh pressure”,Appl. Phys. Lett., vol. 61, p. 1951–1953 (1992)) by using the Young'smodulus in the literature (Nagatsuma, K., Ito, Y., Jyomura, S. Takeuchi,H. and Ashida, S., Piezoelectricity, in FERROELECTRICITY AND RELATEDPHENOMENA. 4, p. 167–176 (Gordon and Breach Science Publications, London(1985)), and Jaffe, B., Cook, W. R. and Jaffe, H., Non-Perovskite OxidePiezoelectrics and Ferroelectrics, in PIEZOELECTRIC CERAMICS, p. 70–74(ACADEMIC PRESS, New York (1971)), and thus the BSO crystals receive acompression stress as large as 12 GPa although the stress caused bycontraction of the platinum electrode is not considered.

In the case where the sandwich structure shown in FIG. 6( b), on theother hand, the compression stress is released by slipping effect atinterfaces therebetween, and the structural change in the BIT—BSO filmdoes not occur.

In the XRD pattern of the BSO—BIT crystals shown in FIG. 5( b), a largepeak shift of from 0.3° to 1° can be confirmed in comparison to the BITcrystals having an accumulated structure shown in FIG. 5( c). It isunderstood from the peak shift amount that the BIT—BSO crystals suffervolume contraction of about 8% in comparison to BIT. It has been knownthat it is necessary to apply a compression stress of from 20 to 30 GPais applied to form such volume compression (J. Haines, et al., PhysicalReview B, vol. 58, p. 2909–2912 (1998), J. Haines and J. M. Leger,Physical Review B, vol. 55, p. 11144–11154 (1997), and B. B. Karki, etal., Physical Review B, vol. 55, p. 3465–3471 (1997)).

That is, in the BIT—BSO structure in the crystallization process, BIT iscrystallized by BSO at a low temperature upon heating, and upon cooling,the Bi₄(Ti,Si) ₃O₁₂ (BSO—BIT) solid solution is formed owing to thedifference in thermal expansion between BSO and BIT. When Si atoms forma perovskite structure, it becomes a perovskite crystal having largecovalency owing to the high covalency of Si, so as to form highresistance to hydrogen that has not been owned by the conventionalferroelectric materials.

Sol-Gel Solution

As the liquid used for forming the oxide material thin film by thesol-gel process, those obtained by dissolving a metallic alkoxide, anorganic salt or an organic base in an organic solvent, such as analcohol, are preferably used. Among these, a metallic alkoxide ispreferably used because it has a constant vapor pressure, can beobtained as a high purity material through a heating and reflux(distillation) process, can be easily dissolved in an organic solvent,forms hydroxide gel or precipitate through reaction with water, andforms a metallic oxide through a baking process in an oxidationatmosphere. The organic solvent for preparing the sol solution ispreferably an alcohol, such as n-butanol, n-propanol and2-methoxyethanol, because they can be well mixed and do not promotefurther polycondensation because they are anhydrous solvents.

The metallic element constituting the metallic alkoxide may be anyelement as far as it is such an element that constitutes the IV Groupelement-containing catalytic substance or the perovskite or lamellarperovskite structure oxide, and examples thereof include an alkalimetal, such as K, Li and Na, an alkaline earth metal, such as Ba, Ca, Mgand Sr, the III Group, such as Al, B and In, the IV Group, such as Si,Ge and Sn, the V Group, such as P, Sb and Bi, a transition element, suchas Y, Ti, Zr, Nb, Ta, V and W, a lanthanoid, such as La, Ce and Nd, andthe like.

In the case where a metallic alkoxide is used as a starting material, itis advantageous because the “n=molecular length (size)” can becontrolled by controlling the polycondensation through partialhydrolysis as described below. That is, the polycondensation reactionitself of the metallic alkoxide can be controlled by adding a knownamount of water.

The gel solution containing the polycondensation product thus obtainedis coated on a substrate by such a method as a spin coating method, adoctor blade method and a spraying method to obtain a thin film.

In this example, for the system of a metallic alkoxide and a metallicsalt of a carboxylic acid for forming PZT, descriptions will be made byusing lead acetate ((CH₃CO₂)₂Pb.H₂O) as a starting material of Pb(lead), titanium tetraisopropoxide (((CH₃)₂CHO)₄Ti) as a startingmaterial of Ti (titanium), zirconium tetra-n-butoxide ((CH₃(CH₂)₃O)₄Zr)as a starting material of Zr (zirconium), and 2-methoxyethanol(CH₃O(CH₂)₂OH) as a solvent.

FIG. 9 is a flow chart of synthesis of a sol-gel solution for forming aPZT ferroelectric thin film. By subjecting alkoxides of Pb, Zr and Tiinstead of that of Si to mixed polycondensation (partial hydrolysis) inthe foregoing chemical formulae, a so-called PZT ferroelectric materialforming sol-gel solution is obtained, in which the respective elementsare bonded through oxygen atoms.

The polymerization degree of the hydrolysis polycondensation product canbe controlled by adjusting the amount of water added as described in theforegoing, and therefore, 3H₂O present as crystallization water in(CH₃CO₂)₂Pb.3H₂O is removed, i.e., it is heated along with CH₃O(CH₂)₂OHas the solvent, whereby water is distillated through azeotropy with thesolvent. A viscous liquid obtained through distillation has such achemical structure CH₃CO₂PbO(CH₂)₂OCH₃.XH₂O (X<0.5) that is obtained bysubstituting one of the acetate groups (CH₃CO₂—) of (CH₃CO₂)₂Pb.3H₂Owith a 2-methoxyethoxy group (CH₃O(CH₂)₂O—). Upon the substitutionreaction, acetic acid (CH₃CO₂H) and an ester of acetic acid andCH₃O(CH₂)₂OH(CH₃CO₂ (CH₂)₂OCH₃ and water (H₂O)) are formed.(CH₃CO₂)₂Pb.3H₂O+CH₃O(CH₂)₂OH→CH₃CO₂PbO(CH₂)₂OCH₃.XH₂O(X<0.5)

((CH₃)₂CHO)₄Ti is then dissolved in CH₃O(CH₂)₂OH to cause the followingalcohol exchange reaction, and all or part of the isopropoxy group((CH₃)₂CHO—) of ((CH₃)₂CHO)₄Ti present as a 1.4-molecular body in theabsence of the solvent is substituted by a 2-methoxyethoxy group.((CH₃)₂CHO)₄Ti+nCH₃O(CH₂)₂OH→((CH₃)₂CHO)_(4−n)Ti(O(CH₂)₂OCH₃)_(n)(n=1 to4)

The similar alcohol exchange reaction occurs in the case where(CH₃(CH₂)₃O)₄Zr is dissolved in CH₃O(CH₂)₂OH.(CH₃(CH₂)₃O)₄Zr+CH₃O(CH₂)₂OH→(CH₃(CH₂)₃O)_(4−n)Zr(O(CH₂)₂OCH₃)_(n)(n=1to 4)

These three liquids are mixed, and water in a measured amount is addedto the molecules to control hydrolysis, whereby a sol-gel solution forforming a PZT ferroelectric thin film is obtained. The mixing ratios ofthe alkoxides of the respective elements and the like may beappropriately adjusted under consideration of the composition of theferroelectric material to be obtained.

The following polymer is present in the solution.

BSO gel shown in FIG. 10 is formed by using the hydrolysis andpolycondensation processes that are similar to the gel for forming a PZTferroelectric thin film.

There is an organic metal decomposition process as a counterpart of thesol-gel process. In this process, a metallic alkoxide and an organicacid salt are dissolved in an organic solvent, such as toluene (C₆H₅CH₃)and xylene (C₆H₄(CH₃)₂), and the liquid is coated on a substrate,followed by subjecting to thermal decomposition, to obtain an oxide thinfilm. Because no water is added in this process, the metallic alkoxideand the organic acid salt thus added are present as they are in thesolution, but do not suffer polycondensation. However, there are someones, such as SBT, that are present as polycondensation products throughoccurrence of a ligand exchange reaction.

For example, a solution for forming an SBT ferroelectric thin film is aso-called MOD solution prepared according to FIG. 11. It is consideredthat a gel structure of a Bi element and a Ta element beinginterdigitated with each other through the carbonyl group (—CO—) of thecarboxylic acid salt, and Sr is present in the spaces thereof, i.e., itsuffers polycondensation. Therefore, it falls within the scope of thepolycondensation gel (sol-gel solution) according to the invention.

As described in the foregoing, the BIT, SBT or PZT solution for forminga ferroelectric thin film obtained by polycondensation with controlledhydrolysis and the BSO solution for forming the IV Groupelement-containing catalytic substance obtained by polycondensation withcontrolled hydrolysis are mixed at room temperature. The mixing ratio ofthe gel solution for forming the ferroelectric material and the gelsolution for forming the catalytic substance can be appropriatelyadjusted depending on the compositions of the solutions, thecharacteristics of the oxide material to be finally obtained, the bakingtemperature, and the like.

For example, in the case where a Bi₂SiO₅ (BSO) sol-gel solution usingn-butanol (specific gravity: 0.813) as a solvent is mixed with aBi₄Ti₃O₁₂ (BIT) sol-gel solution using n-propanol (specific gravity:0.79) as a solvent at a ratio of R=0.4 to form a raw material solution,assuming that all the concentrations are 10% by weight, the gram numbersof BSO and BIT contained in the solvents are 1,000×0.813=813 g for BSOand 1,000×0.79=790 g for BIT. The molecular weights of BSO and BIT perone mole except for oxygen are 446.0455 for BSO and 979.56 for BIT, andthus, the molar concentrations thereof are 1.8 mole/L for BSO and 0.81mole/L for BIT. Therefore, in the case where they are mixed to a mixedsol-gel solution of R=0.4, BSO is mixed at a ratio of 0.4, and thus itis sufficient that 850 ml of the BIT sol-gel solution is added to 150 mlof the BSO sol-gel solution.

The solutions are preferably those after completing thepolycondensation. The solutions are in an anhydrous state, and thus, nofurther hydrolysis proceeds even upon mixing, whereby considerablystable raw material solutions are obtained.

State of Water in Sol-Gel Solution

An airtight container containing a BLT sol-gel solution of 5% by weightand a BSO sol-gel solution of 1% by weight, which have been prepared inthe same manner as in the foregoing was opened in a draft chamber in aclean room for about one hour, and 24 hours after closing the container,a thin film of FIG. 6( a) was formed under the same conditions as in theforegoing.

In the case as described in the foregoing where water content in the airis intentionally introduced into the mixed sol-gel solution, and then itis used for forming a thin film, it is understood as shown in FIG. 12that a paraelectric pyrochlore layer grows, and the function of BSO isfailed. As a result, the ferroelectric characteristics are significantlypoor as shown in FIG. 13.

This is because hydrolysis proceeds with water content in the air tofail to retain independent networks of BSO and BIT in the mixed sol-gelsolution, and thus the structure shown in FIG. 4 cannot be maintained.Accordingly, it is important that the sol-gel solution for forming theIV Group element-containing catalytic substance and the sol-gel solutionfor forming the ferroelectric material are mixed in an anhydrous state,i.e., such a solution is used as a raw material solution in that the gelcontaining the IV Group element connected through oxygen and the gelcontaining the element constituting the ferroelectric material connectedthrough oxygen are dispersed in the same solution and are presentindependently.

EXAMPLE 3

Characteristics of Solid Solution Thin Film of BSO—Bi₄Ti₃O₁₂ (BIT),BSO—SrBi₂Ta₂O₉ (SBT) and BSO—PbZr_(0.52)Ti_(0.48)O₃ (PZT)

A BIT, SBT or PZT solution for forming a ferroelectric thin film, thepolycondensation of which has been completed with controlled hydrolysis,and a BSO solution for forming a dielectric material thin film, thepolycondensation of which has been completed with controlled hydrolysis,are mixed at room temperature according to Example 2. The BSO gelcontent in the mixed gel for forming a ferroelectric material was amolar ratio R=0.4, 0.33 and 0.1 for BIT, SBT and PZT, respectively. Theraw material gel is one having the stoichiometric composition.

The sol-gel mixed solution was coated on a Pt/Ti/SiO₂/Si substrate by aspin coating method, and a film was formed under the following filmforming conditions. The thickness was 100 nm in all the cases.

(Conditions for Forming Ferroelectric Thin Film)

(1) Spin coating (at 500 rpm for 5 seconds and 4,000 rpm for 20seconds), (2) drying (in the atmosphere at 50° C. for 2 minutes), (3)calcination (in the atmosphere at 400° C. for 5 minuets), andsubsequently (4) baking (crystallization) (at 450 to 700° C. for 10minutes, RTA in oxygen) were carried out.

FIG. 14 shows the XRD patterns of the conventional BIT and the BSO—BIT(R=0.4) according to the invention, having a thickness of 100 nm andbeing produced by the foregoing process.

The BIT having BSO added thereto according to the invention exhibitsgood crystallinity at 500° C. On the other hand, BIT containing no BSOcannot provide a single layer at 700° C., and it is understood that adielectric pyrochlore layer (Bi₂Ti₂O₇) and BIT are present as a mixture.

It is understood from the foregoing that the crystallization temperatureof the BSO-added BIT according to the invention is lowered by about 200°C. in comparison to the conventional BIT.

The surface observation of the conventional BIT and the BSO-added BITaccording to the invention reveals that the conventional BIT has aconsiderably coarse surface state although substantially nocrystallization peak is found at 600° C., whereas the BSO-added BITaccording to the invention has a dense and smooth film surface.

The observation of the interface between platinum and the ferroelectricmaterial with TEM cross sectional images thereof reveals that thepresence of an amorphous layer having a thickness of about 5 nm isconfirmed in the conventional BIT, whereas the interface in theBSO-added BIT according to the invention has no presence of an amorphouslayer to provide an interface in good conditions.

These results show that the crystalline growth mechanisms of the twocases are completely different from each other.

That is, in the case of the conventional BIT, BIT crystalline initialnuclei are formed only at the interface to the platinum electrode, andthey grow upward and along the platinum interface as shown in FIG. 15(a), so as to provide a film surface after growth having convex upwardcoarse surface morphology.

On the other hand, in the case of the BSO-added BIT according to theinvention, BIT crystalline initial nuclei form on all the fiveinterfaces upon forming the amorphous state before crystallization(since four-layer coating is carried out on the Pt substrate as shown asthe forming conditions, there are five interfaces including an interfaceto the Pt electrode and those among the respective layers) as shown inFIG. 15( b). In particular, there is no amorphous layer at the interfaceto the Pt electrode. It is considered that the smooth surface morphologyis obtained as a result. This phenomenon is largely contributed by BSO,crystallization of which starts at a low temperature of 450° C. (Kijima,T. and Matsunaga, H., “Preparation of Bi₄Ti₃O₁₂ Thin Film on Si (100)Substrate Using Bi₂SiO₅ Buffer Layer and Its Electric Characterization”,Jpn. J. Appl. Phys., vol. 37, p. 5171–5173 (1998)).

As shown in the XRD patterns of FIG. 16, the decrease in crystallizationtemperature and the considerable improvement in surface morphology arefound in BSO-added SBR (R=0.33) having been baked at 550° C. for 10minutes and BSO-added PZT (R=0.1) having been baked at 450° C. for 10minutes, as similar to BIT. In the case of the conventional PZT, anamorphous layer having a low dielectric constant is formed at theinterface to the platinum electrode.

The resulting ferroelectric thin films were measured for P-V hysteresischaracteristics by forming a platinum upper electrode having a diameterof 100 μm thereon.

As a result, the BSO-added BIT, SBT or PZT ferroelectric thin filmaccording to the invention exhibits good hysteresis characteristicsshown in FIG. 17 although the baking temperature is lower by about from150 to 200° C. than the conventional BIT, SBT or PZT ferroelectric thinfilm. The conventional BIT, SBT or PZT ferroelectric thin film, on theother hand, does not exhibit hysteresis characteristics with a thicknessof 100 nm due to deteriorated leakage characteristics, which isconsidered to be ascribed to the coarse surface morphology thereof.

The resulting ferroelectric characteristics are as follows. TheBSO-added BIT at 500° C. exhibits a residual polarization Pr=17 μC/cm²and a coercive electric field Ec=95 kV/cm, and the BSO-added BIT at 600°C. exhibits a residual polarization Pr=21 μC/cm² and a coercive electricfield Ec=95 kV/cm. The BSO-added SBT at 550° C. exhibits Pr=7 μC/cm² andEc=50 kV/cm, and the BSO-added SBT at 600° C. exhibits Pr=11 μC/cm² andEc=60 kV/cm. BSO-added PZT at 450° C. exhibits Pr=20 μC/cm² and Ec=45kV/cm, and the BSO-added PZT at 550° C. exhibits Pr=25° C./cm² and Ec=38kV/cm.

As understood from the foregoing results, the ferroelectric materialhaving BSO added thereto according to the invention has such an effectthat the characteristics of the original ferroelectric material can beutilized as much as possible without any modification thereof throughthe decrease in crystallization temperature and the significantimprovement in surface morphology.

Evaluation results of fatigue characteristics are then shown in FIG. 18.When a pulse electric field of an application voltage of 3 V and afrequency of 100 kHz is applied to carry out 10¹⁰ times polarizationinversions, such considerably good fatigue characteristics are obtainedfor all the ferroelectric capacitors that the decrease in polarizationvalue is 3% or less. It is considered that this is because of the goodcrystallinity, the smooth film surface, the good interface containing nohetero-phase, and the like. In particular, while it has been known thatPZT suffers fatigue on a Pt electrode, the BSO-added PZT according tothe invention provides good fatigue characteristics as similar to BIT.

Upon further consideration of the difference between the invention andthe conventional art, it has been found that not only the mechanisms ofcrystal growth are different, but also there is a difference incrystalline structure itself.

It is apparent from observation of the TEM cross sectional images of theBSO-added BIT according to the invention and the conventional BIT thatother differences than the B site ion replacement occurs between them.

In the conventional BIT, the c axis length of BIT is 32 Å or less, whichwell agrees with the bulk value in FIG. 19. It is understood that thereis such a structure that has BIT regularly arranged in the transversaldirection in the figure.

In the BSO-added BIT according to the invention, on the other hand, itis understood that the c axis length of BIT is as short as 31 Å.Furthermore, as different from the conventional BIT, such a complexstructure is confirmed in that it is not arranged in the transversaldirection in the figure to form partial vertical deviations, and BSO isintroduced into the deviated parts. The c axis length of BSO is 15 Å orless, which substantially agree with the bulk data in FIG. 20.

It is understood from the foregoing that the BSO-added BIT according tothe invention has the structure shown in FIG. 21. This evidences thatBSO accelerates crystallization of BIT. Furthermore, the deviation incrystallization reflects the growing mechanism of silicate, and thesurface area of silicate is increased owing to the structure to exertgood functions.

There is no practically used polycrystalline Pt substrate having acompletely flat surface without unevenness, and thus a large amount ofunevenness is present on the ferroelectric thin film formed thereon.Therefore, when the BIT initial nuclei formed at arbitrarily positionsare grown, there is substantially no case where adjacent ones areconformably grown as connected to each other, and the crystal growth isinhibited at the part where the crystals are not connected to eachother. According to the structure of the invention, on the other hand,even when certain bumps are present, such an effect can also be obtainedthat the ferroelectric crystals are grown as connected to each other toabsorb the bumps.

Furthermore, the fact that the crystalline structure shown in FIG. 20provides good ferroelectric characteristics even when dielectric BSO iscontained can be explained as follows.

In general, there is substantially no case where the used substrate andthe thin film thereon are completely agree with each other in latticematching, and there are lattice deformation and stress in the thin film.Further, the ferroelectric material used in the invention is such aferroelectric material that is referred to as a displacement type, whichsuffers displacement upon polarization to form stress.

On the other hand, BSO has the accumulated structure of Bi₂O₃ layers andSiO₂ layers as shown in FIG. 20. That is, the silicon oxide layer hassuch a structure that the tetrahedral structure of silicon oxide ishorizontally aligned with the bismuth oxide layers. Silicon oxide is agood piezoelectric material and causes polarization by externalpressure. When the silicon oxide is polarized (BSO is polarized) due tothe stress in the film caused by the lattice inconformity and the like,good characteristics can be obtained without deterioration of theferroelectric characteristics of BIT due to BSO.

Furthermore, when a Pt/BIT/Pt capacitor according to the invention isannealed in N₂ containing 3% of H₂ at 400° C., good reduction resistanceas shown in FIG. 22 is exhibited. It is considered that this is becausea part of B site Ti in BIT is replaced by Si in BSO to increase thecovalency of BIT, and it is considered that the c axis length of BIT ischanged as a result of the B site replacement and the formation of thesolid solution with BSO.

The fact that the solid solution thin film of BSO and BIT according tothe invention contains a large amount of Si has been revealed as a cleardifference. It has generally known that in the case where a large amountof Si element is mixed in a ferroelectric film by diffusing from an Sisubstrate, the diffused Si causes increase of a leakage electric currentdensity even though a not large amount thereof is mixed therein. In thecase of an Si substrate having a metallic electrode, such as Pt, coveredthereon, it is covered with a thermal oxidation SiO₂ film having athickness of about 200 nm to improve the adhesion between Pt and the Sisubstrate, and thus the SiO₂ film is considerably stable. Therefore, itis difficult that Si is diffused into the ferroelectric film unless thebaking temperature upon producing the ferroelectric thin film is toohigh. In the case of the conventional BIT, actually, Si is not diffusedinto the ferroelectric thin film at a baking temperature of 700° C.

Although the solid solution thin film of BSO and BIT according to theinvention contains a large amount of Si in the film, considerablyexcellent ferroelectric characteristics are confirmed. Thus, the Si inthe film is not one diffused from the substrate, but Si in BSO appearsupon analysis. The fact can easily distinguish the conventional art andthe invention through a great difference.

Comparison in Characteristics Between BIT—BSO Solid Solution Thin Filmand BIT—Bi₄Si₃O₁₂ Solid Solution Thin Film

A mixed solution formed by mixing a Bi₄Si₃O₁₂ sol-gel solution having aconcentration of 10% by weight with a BIT sol-gel solution having aconcentration of 10% by weight at a ratio of R=0.33 was used as a rawmaterial solution for forming a BIT—B₄Si₃O₁₂ solid solution.

The sol-gel solution was coated on a Pt/Ti/SiO₂/Ti substrate by a spincoating method, and a ferroelectric thin film having a thickness of 100nm was formed under the following film formation conditions.

(Ferroelectric Thin Film Formation Conditions)

A series of operations, (1) spin coating (at 500 rpm for 5 seconds and4,000 rpm for 20 seconds), (2) drying (in the atmosphere at 150° C. for2 minutes) and (3) calcination (in the atmosphere at 400° C. for 5minutes) were repeated four times, and subsequently (4) baking(crystallization) (at 600° C. for 10 minutes, RTA in oxygen) was carriedout.

XRD patterns at this time is shown in FIG. 23. Comparison was made byusing XRD patterns of the BIT+BSO of R=0.4 produced in the foregoingexample and the conventional BIT. The baking temperature for theBi₄Si₃O₁₂-added BIT and the BIT+BSO according to the invention was 600°C., and that for the conventional BIT was 700° C.

In the case of the Bi₄Si₃O₁₂-added BIT and the BIT+BSO according to theinvention, all the peaks are shifted to the side of large angles incomparison to the conventional BIT, and the crystals of theBi₄Si₃O₁₂-added BIT and the BIT+BSO have a contracted structure bycompressing by about 8% (2%×2%×2%) over the entire crystals incomparison to the BIT. This exhibits the presence of a considerableamount of stress in the film.

According to the results of the compression experiments of SnO₂, GeO₂and the like at high temperatures (B. B. Karki, et al., Physical ReviewB, “Ab initio studies of high-pressure structural transformation insilica”, vol. 55, p. 3465–3471 (1997); J. Haines, et al., PhysicalReview B, “Phase transition in ruthenium dioxide up to 40 GPa:Mechanisms for the rutile-to-fluorite phase transformation and a modelfor the high-pressure behavior of stishovite SiO₂”, vol., 48, p.13344–13350 (1993); and J. Haines, et al., Physical Review B, “X-raydiffraction study of the high-pressure: Relationships between structuretypes and implications for other rutile-type dioxides”, vol.55,p.11144–11154 (1997)), it has been exhibited that a compression stressof from 25 to 30 GPa is necessarily applied to the entire film in orderto compress the volume of the ferroelectric crystals by about 8%, andthe equivalent compression stress is applied to the thin film of theinvention.

The measurement of hysteresis characteristics of the Bi₄Si₃O₁₂-added BITrevealed that good square hysteresis characteristics were obtainedalthough the hysteresis shape was somewhat deteriorated. The evaluationof film fatigue characteristics of the Bi₄Si₃O₁₂-added BIT revealed thatno film fatigue was found upon 10¹⁰ times polarization inversions.

The crystallization temperature is decreased by about 200° C. in theforegoing example. This is because of such an effect that thecrystallization temperature of Bi₂SiO₅ (BSO) is as low as 400° C., andthe lattice matching is good between the Bi oxide layer in the BSOlattice and the perovskite layer and the pseudo-perovskite layer of theferroelectric perovskite containing ABO₃ or(Bi₂O₂)²⁺(A_(m−1)B_(m)O_(3m+1))²⁻ and the bismuth lamellar structureferroelectric material, whereby the ferroelectric perovskite and thebismuth lamellar structure ferroelectric material are crystallized withBSO as crystalline nuclei.

Not only Si—O has high covalency, and Si is not a metallic element, butalso it has been known that for an atom for replacing the constitutionalelement of perovskite, one having an ionic radius that is closer theretois used. Si⁴⁺ is 0.26 Å, which is far smaller than Ti⁴⁺ of 0.75 Å, andthus it is difficult to nominate Si⁴⁺ as an ion for replacing the B siteion under normal consideration. However, the replacement of the B siteions in the ferroelectric material with Si⁴⁺ ions solves the problemsassociated with the conventional ferroelectric thin film.

Use of Ge-Containing Catalytic Substance

In this example, a ferroelectric material having B site ions replacednot with Si but with Ge, which is a related element of Si, i.e., theferroelectric material having B site ions replaced with G⁴⁺ but notSi⁴⁺, was evaluated.

An Si substrate having Pt coated to a thickness of 100 nm was used asthe substrate, a solution obtained by mixing a Bi₄Ti₃O₁₂ sol-gelsolution containing 16% of Bi₄Ge₃O₁₂ is coated on the substrate to forma thin film having a thickness of 100 nm under the following filmformation conditions.

(Ferroelectric Thin Film Formation Conditions)

A series of operations, (1) spin coating (at 500 rpm for 5 seconds and4,000 rpm for 20 seconds), (2) drying (in the atmosphere at 150° C. for2 minutes) and (3) calcination (in the atmosphere at 400° C. for 5minutes) were repeated four times, and subsequently (4) baking(crystallization) (at 550° C. for 30 minutes, RTA in oxygen) was carriedout.

At this time, AFM observation of the film surface revealed that it hadsignificantly good surface smoothness, i.e., Rmax, which indicated themost coarse part, was 2.001 nm, and Ra, which indicated smoothness overthe entire film, was 1.5022 nm.

A Pt upper electrode is formed, and evaluation of theBi₄Ge₃O₁₂—Bi₄Ti₃O₁₂ ferroelectric thin film having a thickness of 100 nmaccording to the invention was carried out for ferroelectriccharacteristics by using the upper Pt and the lower Pt. The results areshown in FIG. 24. The D-E hysteresis characteristics was as good asPr=19 μC/cm².

The fatigue characteristics were also evaluated. The results thereof areshown in FIG. 25. Substantially no film fatigue was found after 10¹⁰times repeated polarization inversions, and it was found that goodfatigue characteristics were exhibited.

As described in the foregoing, it was found that Ge⁴⁺ exhibits theeffect of replacement of the B site ions as similar to Si⁴⁺. Therefore,Bi₂GeO₅ can be used as similar to Bi₂SiO₅. Solid Solutions of Bi₂SiO₅(BSO) and Bi₄Ti₃O₁₂ (BIT), and Bi₂SiO₅ (BSO) and SrBi₂Ta₂O₉ (SBT)

BIT having an LSO sol-gel solution using n-propanol as a solvent addedthereto at R=0.4, and SBT having the same added thereto at R=0.33 werecoated by a spin coating method to a thickness of 25, 50 or 100 nm.

An ultrathin film having a thickness of 10 nm was simultaneously formedby using a Bi_(3.25)La_(0.75)Ti₃O₁₂ (BLT) sol-gel solution having BSO atR=0.4.

(Ferroelectric Thin Film Formation Conditions)

A series of operations, (1) spin coating (at 500 rpm for 5 seconds and4,000 rpm for 20 seconds), (2) drying (in the atmosphere at 150° C. for2 minutes) and (3) calcination (in the atmosphere at 400° C. for 5minutes) were repeated four times, and subsequently (4) baking(crystallization) (BIT: at 550° C., SBT: at 600° C., for 10 minutes, RTAin oxygen) was carried out.

In the case of a thickness of 25 nm, coating was once carried out. Inthe case of a thickness of 50 nm, coating was twice carried out. In thecase of a thickness of 100 nm, coating was four times carried out. Inthe case of the 10 nm BLT thin film, coating was once carried out, andthe rotation number upon coating was increased to 7,000.

Subsequently, an upper Pt electrode (100 μm) was formed as similar tothe foregoing example, and P-E hysteresis characteristics wereevaluated. As shown in FIGS. 26 and 27, good hysteresis characteristicswere obtained.

As shown in FIG. 26, the hysteresis characteristics upon applying thecommon applied electric field of 500 kV/cm were Pr=15 to 18 μC/cm² andEc=100 kV/cm or less for BIT, and Pr=11 μC/cm² or less and Ec=60 to 70kV/cm for SBT, which showed good agreements.

As shown in FIG. 27, in the case where the applied voltage was scaled onthe abscissa axis, the coercive voltage for a thickness of 100 nm, 50 nmand 25 nm was 1 V, 0.5 V and 0.25 V for BIT and 0.7 V, 0.35 V and 0.2 Vfor SBT, which changed in substantial proportion.

The hysteresis characteristics of the 10 nm BSO-BLT are shown in FIG.28. It was saturated at an applied voltage of 0.5 V and was Pr=16 μC/cm²and Ec=100 kV/cm, and Pr=17 μC/cm² and Ec=120 kV/cm at an appliedvoltage of 0.7 V. In other words, it shows that a memory element thatcan be used at a driving voltage of 0.5 V can be provided by usingBSO-BLT having a thickness of 10 nm.

It is understood from the foregoing that reduction in thickness iseffective to lower the driving voltage of the ferroelectric memory. Thatis, such an extent of reduction in thickness of a ferroelectric materialcan be realized that has not yet been attained, and a ferroelectricultrathin film for a highly integrated memory of a Mbit level can beobtained.

This is an effect of forming crystalline nuclei on all the interfacespresent in the film until the calcination step by using the invention,and because the uppermost interface among these can be effectively usedfor crystallization of the ferroelectric thin film, dense and flatsurface morphology, which is most important for reducing the thickness,can be provided.

Ferroelectric Characteristics Under Change of Amount of IV GroupElement-containing Catalytic Substance in Solid Solution

A sol-gel solution formed by adding LSO to a BIT sol-gel solution atR=0.2 was used as a base, and BSO gel was further added thereto toproduce five kinds of mixed sol-gel solutions of R=0.2 (BSO=0), R=0.4(BSO=0.2), R=0.8 (BSO=0.6) R=1.6 (BSO=1.4) and R=3.2 (BSO=3.0).

The mixed sol-gel solution was coated on a Pt/Ti/SiO₂/Si substrate by aspin coating method to form a ferroelectric thin film under thefollowing film forming conditions.

The backing conditions were at 500° C. for 10 minutes, and the thicknesswas 100 nm in all the cases.

(Ferroelectric Thin Film Formation Conditions)

A series of operations, (1) spin coating (at 500 rpm for 5 seconds and4,000 rpm for 20 seconds), (2) drying (in the atmosphere at 150° C. for2 minutes) and (3) calcination (in the atmosphere at 400° C. for 5minutes) were repeated four times, and subsequently (4) baking(crystallization) (at 500° C. for 15 minutes, RTA in oxygen) was carriedout.

The XRD pattern of the BIT film of R=0.4 (BSO=0.2) revealed goodcrystallinity at a substrate temperature of 500° C. as shown in FIG. 29.

The hysteresis characteristics of five kinds of BIT capacitorscrystallized at 500° C. show such a change that Pr was decreasedinversely proportional to decrease of R as shown in FIG. 30.

In the case where a dielectric material is added to a ferroelectricmaterial, conventionally, it often adversely affects only with anaddition of several percents. This is because many of dielectricmaterials have a smaller dielectric constant than a ferroelectricmaterial, and thus almost the applied voltage is applied to thedielectric layer added in an amount of several percents, whereby goodferroelectric characteristics cannot be obtained.

In the invention, however, in the case where BSO or LSO is added to aferroelectric material, the characteristics of the ferroelectricmaterial used as a base can be obtained to the maximum extent, and mostof the problems associated with the conventional ferroelectric materialscan be removed.

When BSO or LSO is added to R=1.6 beyond the molar concentration of theferroelectric material as a base, various kinds of ferroelectriccharacteristics with different Ec and Pr can be obtained.

Furthermore, in the case of R=3.2 where almost the entire body isconstituted with a dielectric material, it is found that a highdielectric film having a dielectric constant of about 200 is obtainedalthough ferroelectricity is not exhibited.

In addition to the effect of realization of the ultrathin film in theforegoing example, the invention can provide such a thin film that hasarbitrary dielectric characteristics depending on purposes, i.e., from agood dielectric material to a capacitor material for DRAM and a highdielectric gate oxide film material for a ultrafine transistor.

That is, as described in the foregoing example, the values of Ec and Prcan be arbitrarily controlled in the case where BSO or LSO is added inan amount beyond the molar concentration of the ferroelectric materialas a base, which evidences the formation of a solid solution.

The ferroelectric characteristics of PZT (Zr/Ti=52/48) having BSO addedin a ratio R=0.025, 0.05, 0.1 and 0.2 were measured.

The film formation was carried out under the following conditions.

A series of operations, (1) spin coating (at 500 rpm for 5 seconds and4,000 rpm for 20 seconds), (2) drying (in the atmosphere at 150° C. for2 minutes) and (3) calcination (in the atmosphere at 400° C. for 5minutes) were repeated four times to obtain a thickness of 100 nm, andsubsequently (4) baking (crystallization) (at 500° C. for 10 minutes,RTA in oxygen at 1 kg/cm²) was carried out. Furthermore, an upperplatinum electrode is formed, and (5) post annealing (at 500° C. for 5minutes, RTA in oxygen at 1 kg/cm² or 2 kg/cm²) was carried out.

As seen by the XRD pattern shown in FIG. 31, the addition of a smallamount, R=0.025, of BSO exhibited good crystallinity.

Good surface morphology was confirmed in all the cases, which revealedthat PZT crystalline nuclei were formed at a high density over theentire film.

Upon measurement of hysteresis characteristics, various kinds ofhysteresis characteristics as similar to BIT were confirmed depending onthe addition amounts as shown in FIG. 32. This evidences that PZT andBSO form a solid solution.

Finally, in the case where the calcination temperature is 300° C., BSOis not crystallized, but amorphous BSO, from which only the organiccomponents are removed, can be utilized.

In the case of a bismuth ferroelectric material, the Bi₂O₃ layer in BSOcan be utilized as a part of the Bi₂O₃ layer of the bismuth lamellarstructure ferroelectric material, but in the case of PZT, which is aperovskite ferroelectric material, it is necessary that Bi is introducedinto A sites.

Introduction of Bi into A sites of a crystallographic system from anamorphous phase is easier than transformation of a certaincrystallographic system to a different crystallographic system. Anamorphous phase has regularity among constitutional atoms, and itsufficiently exerts such a function that selectively crystallizes PZT.Furthermore, upon comparison of crystals, the amorphous phase has alarger surface area, and thus it has been exhibited that the amorphousphase sufficiently has a catalytic effect.

EXAMPLE 4

In this example, solid solution thin films of various kinds of catalyticsubstances with BIT, SBT and PZT were formed.

All the sol-gel solutions for forming a ferroelectric thin film had aconcentration of 5% by weight, and all the sol-gel solutions for forminga catalytic substance had a concentration of 0.3% by weight. Both thesolutions were mixed to form a raw material solution. The sol-gelsolution was applied on a Pt/Ti/SiO₂/Si substrate by a spin coatingmethod to form a thin film having a thickness of 20 nm under thefollowing film formation conditions. The mixing molar ratios were R=0.4for BIT, R=0.33 for SBT, and R=0.2 for PZT.

(Thin Film Formation Conditions)

A series of operations, (1) spin coating (at 500 rpm for 5 seconds and4,000 rpm for 20 seconds), (2) drying (in the atmosphere at 150° C. for2 minutes) and (3) calcination (in the atmosphere at 400° C. for 5minutes) were repeated four times to obtain a thickness of 100 nm, andsubsequently (4) baking (crystallization) (at 600° C. for 15 minutes,RTA in oxygen at 1 kg/cm²) was carried out. An upper platinum electrodeis formed, and (5) post annealing (at 500° C. for 5 minutes, RTA inoxygen at 1 kg/cm² or 2 kg/cm²) was carried out.

All the thin films obtained herein were crystalline films having athickness of 70 nm and exhibited good ferroelectric characteristics asshown in FIGS. 33 to 35.

EXAMPLE 5

A BSO sol-gel solution having a concentration of 0.1% by weight wasmixed with a BST sol-gel solution having a concentration of 5% by weightat a ratio R=0.2 to form a raw material solution. The sol-gel solutionwas applied to a Pt/Ti/SiO₂/Si substrate by a spin coating method toform a thin film having a thickness of 20 nm under the following filmformation conditions.

(High Dielectric Thin Film Formation Conditions)

(1) Spin coating (at 500 rpm for 5 seconds and 4,000 rpm for 20seconds), (2) drying (in the atmosphere at 150° C. for 2 minutes) and(3) calcination (in the atmosphere at 400° C. for 5 minutes) werecarried out, and subsequently (4) baking (crystallization) (at 600° C.for 15 minutes, RTA in oxygen at 1 kg/cm²) was carried out. An upperplatinum electrode is formed, and (5) post annealing (at 500° C. for 5minutes, RTA in oxygen at 1 kg/cm²) was carried out.

The thin film obtained herein was a crystalline film having a thicknessof 20 nm in good conditions and had electric characteristics as shown inFIG. 36. A dielectric constant of about 600 was obtained, which wasequivalent to that of a bulk material.

A BSO sol-gel solution having a concentration of 0.3% by weight wasmixed with a Bi2223 sol-gel solution having a concentration of 2% byweight at a ratio R=0.4 to form a raw material solution.

The sol-gel solution was applied to an Si (100) substrate, a spontaneousoxide film of which had been removed, by a spin coating method to form athin film having a thickness of 20 nm under the following film formationconditions.

(Superconductive Thin Film Formation Conditions)

(1) Spin coating (at 500 rpm for 5 seconds and 4,000 rpm for 20seconds), (2) drying (in the atmosphere at 150° C. for 2 minutes) and(3) calcination (in the atmosphere at 400° C. for minutes) were carriedout, and subsequently (4) baking (crystallization) (at 700° C. for 1minute, RTA in oxygen at 1 kg/cm²) was carried out. An upper platinumelectrode is formed, and (5) post annealing (at 500° C. for 5 minutes,RTA in oxygen at 1 kg/cm²) was carried out.

The thin film obtained herein was a crystalline film having a thicknessof 15 nm in good conditions and was constituted with a Bi2223 singlephase as shown in FIG. 37.

BST is expected as a gate oxide film material as an alternative of SiO₂for a next-generation DRAM, and has been continuously studied in recent10 years or more (Kazuhide Abe and Shuichi Komatsu, Jpn. J. Appl. Phys.,vol. 33, p. 5297–5300 (1994)). In a Ba_(0.5)Sr_(0.5)TiO₃ (BST)perovskite high dielectric material and a Bi₂Sr₂Ca₂Cu₃O_(x) (Bi2223)superconductive oxide material, they can be used as a thin film havinggood characteristics by using a solid acid catalytic substance.

EXAMPLE 6

As shown in FIG. 38( a), a BSO—BIT thin film was formed by a sol-gelprocess on a Si substrate having an amorphous Si₃N₄ coating having athickness of 1.8 nm. The ferroelectric thin film had a thickness of 150nm.

As similar to the foregoing, a series of operation, (1) spin coating (at500 rpm for 5 seconds and 4,000 rpm for 20 seconds), (2) drying (in theatmosphere at 150° C. for 2 minutes) and (3) calcination (in theatmosphere at 400° C. for 5 minutes) were repeated three times.Subsequently (4) baking (crystallization) (at 500° C. for 30 minutes,RTA in oxygen at 1 kg/cm²) was carried out, and an upper platinumelectrode was formed.

The resulting BIT—BSO thin film exhibited good crystallinity as shown inFIG. 39, and good diode characteristics were obtained as shown in FIG.40.

On a single crystal Si(100) substrate, a mixed solution of a SBT sol-gelsolution and a BSO sol-gel solution (R=0.33) was subjected to thefollowing operations, i.e., a series of operation, (1) spin coating (at500 rpm for 5 seconds and 4,000 rpm for 20 seconds), (2) drying (in theatmosphere at 150° C. for 2 minutes) and (3) calcination (in theatmosphere at 400° C. for 5 minutes) were repeated three times, andsubsequently (4) baking (crystallization) (at 600° C. for 30 minutes,RTA in oxygen at 1 kg/cm²) was carried out.

Good crystallinity was exhibited at this time as shown in FIG. 41.

An Al/BSO—SBT/Si transistor shown in FIG. 38( b) was then produced, andthus good transistor characteristics were obtained as shown in FIGS. 42and 43.

The oxide material of the invention can be used in an integrated circuitas a part of constitution of an optical modulator, an ultrasonic sensor,an infrared linear sensor, a capacitor for DRAM and MMIC, aferroelectric device or a semiconductor device. For example, it ispossible that a ferroelectric element is applied to a capacitor part ofa nonvolatile memory, or a ferroelectric element is applied to a gate ofan FET, and a gate dielectric film, a source/drain region and the likeare formed in combination, so as to form an MFMIS-FET, an MFS-FET or thelike.

In the case where the oxide material of the invention is utilized in asubstrate or an element by using a thin film production technique, athin film of the oxide material is generally formed on a substrate withor without an electroconductive film intervening therebetween. Examplesof the substrate used herein include a semiconductor substrate, such asan element semiconductor, e.g., silicon and germanium, and a compoundsemiconductor, such as GaAs and ZnSe, a metallic substrate, such as Pt,and an insulating substrate, such as a sapphire substrate, an MgOsubstrate, SrTiO₃, BaTiO₃ and a glass substrate. Among these, a siliconsubstrate is preferred, and a silicon single crystal substrate is morepreferred.

An electroconductive film that may be formed on the substrate is notparticularly limited as far as it is an electroconductive material thatis generally used as an electrode and wiring, and examples thereofinclude a single layer film or a multilayer film of a metal and analloy, such as Pt, Ir, Au, Al and Ru, an oxide electroconductivematerial, such as IrO₂ and RuO₂, a nitride electroconductive material,such as TiN and TaN, and the like. The thickness of theelectroconductive film may be, for example, about from 100 to 200 nm.

An intermediate layer, such as a dielectric layer and an adhesion layer,may be formed between the electroconductive film and the substrate. Thedielectric layer may be formed, for example, with SiO₂, Si₃N₄ or thelike. The material for the adhesion layer is not particularly limited asfar as it can ensure the adhesion strength between the substrate and theelectroconductive film or between the dielectric film and theelectroconductive film, and examples thereof include a high meltingpoint metal, such as tantalum and titanium. The intermediate layer canbe formed by various methods, such as a thermal oxidation method, a CVDmethod, a sputtering method, a vapor deposition method and an MOCVDmethod.

While the oxide material of the invention is preferably formed by thesol-gel process, it may be formed by various methods, such as an MOCVDmethod, a laser abrasion method and a sputtering method.

INDUSTRIAL APPLICABILITY

According to the invention, because a solid solution of a solid acidcatalytic substance and a perovskite or lamellar perovskite structureoxide material as a catalytically active substance is provided, thestrongest covalency can be introduced into the perovskite or lamellarperovskite structure oxide material, and the reactivity in a processconcerning ions, such as active hydrogen (H⁺) and an applied electricfield (e⁻) can be prevented. According thereto, hydrogen deteriorationof the perovskite or lamellar perovskite structure oxide material can beprevented, and the scattering in compositional distribution in theperovskite or lamellar perovskite structure oxide material can beprevented, whereby application to a memory element becomes possible.

The perovskite or lamellar perovskite structure oxide material can beeffectively crystallized at a low temperature owing to the function ofthe solid catalytic substance. Furthermore, inconsistency of lattices ofcrystal grains can be suppressed by the formation of the solid solution,so as to obtain dense and smooth interface and surface morphology, andthus the characteristics of the perovskite or lamellar perovskitestructure oxide material can be utilized as much as possible without anychange thereof. In other words, in the case where the perovskite orlamellar perovskite structure oxide material film is utilized as anelement, the leakage characteristics of the element can be improved toobtain good hysteresis. Moreover, good fatigue characteristics can alsobe obtained owing to the good crystallinity, the smooth film surface,and the good interface without hetero-phase.

In particular, the foregoing effect becomes conspicuous in the casewhere Si⁴⁺, Ge⁴⁺ or Sn⁴⁺ is contained at the position of cations in thecrystalline lattice or the oxygen octahedrons of the perovskite orlamellar perovskite structure oxide material.

According to the invention, furthermore, the gel solution for formingthe catalyst and the gel solution for forming the perovskite or lamellarperovskite structure oxide material are used by mixing, whereby theperovskite or lamellar perovskite structure oxide material having thelow temperature crystallization property, the smooth interface and filmsurface and the like as described in the foregoing can be convenientlyand effectively produced.

1. An oxide material having a perovskite structure comprising an oxiderepresented by a formula selected from the group consisting of ABO₃,(Bi₂O₂)²⁺(A_(m−1)B_(m)O_(3m+1))²⁻, wherein A represents one kind or twoor more kinds of ions selected from the group consisting of Li⁺, Na⁺,K⁺, Pb²⁺, Ca²⁺, Sr²⁺, Bi³⁺, Y³⁺, Mn³⁺ and La³⁺, B represents one kind ortwo or more kinds of ions selected from the group consisting of Ru₃₊,Fe³⁺, Ti⁴⁺, Zr⁴⁺, Cu⁴⁺, Nb⁵⁺, Ta⁵⁺, V⁵⁺, W⁶⁺ and Mo⁶⁺, and m representsa natural number of 1 or more, LnBa₂Cu₃O₇, Z₂Ba₂Ca_(n−1)Cu_(n)O_(2n+4)and ZBa₂Ca_(n−1)Cu_(n)O_(2n+3), wherein Ln represents one kind or two ormore kinds of ions selected from the group consisting of Y, La, Ce, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, Z represents one kindor two or more kinds of ions selected from the group consisting of Bi,Tl and Hg, and n represents a natural number of from 1 to 5; wherein theoxide material is partially substituted with a catalytic substancecontaining Si.
 2. A material as claimed in claim 1, wherein Si⁴⁺ iscontained at the position of cation in the crystalline latticeconstituting the perovskite or lamellar perovskite structure oxide.
 3. Amaterial as claimed in claim 2, wherein Si⁴⁺ is contained at the centerof oxygen octahedron constituting the perovskite or lamellar perovskitestructure oxide.
 4. A material as claimed in claim 1, wherein thecatalytic substance is a complex oxide materials comprising one or morekinds of oxides selected from the group consisting of CaO, PbO, ZnO,SrO, MgO, FeO, Fe₂O₃, B₂O₃, Al₂O₃, In₂O₃, Y₂O₃, Sc₂O₃, Sb₂O₃, Cr₂O₃,Bi₂O₃, Ga₂O₃, CuO₂, MnO₂, ZrO₂TiO₂, MoO₃, WO₃, V₂O₅, and a lanthanoidoxide, and SiO₂.
 5. A material as claimed in claim 4, which is a complexoxide material represented by the formula X₂SiO₅ or X₄Si₃O₁₂ (wherein Xrepresents Ca₂₊, Pb²⁺, Zn²⁺, Sr²⁺, Mg²⁺, Fe²⁺, Fe³⁺, B³⁺, Al³⁺, In³⁺,Y³⁺, Sc³⁺, Sb³⁺, Cr³⁺, Bi³⁺, Ga³⁺, Cu⁴⁺, Mn⁴⁺, Zr⁴⁺, Ti⁴⁺, Mo⁶⁺, V⁵⁺,La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺,Tm³⁺, Yb³⁺, or Lu³⁺).
 6. An oxide forming mixed solution of an anhydrousmixture comprising (i) a polycondensation product for forming aperovskite or perovskite lamellar structure oxide material representedby a formula selected from the group consisting of ABO₃,(Bi₂O₂)²⁺(A_(m−1)B_(m)O_(3m+1))², wherein A represents one kind or twoor more kinds of ions selected from the group consisting of Li⁺, Na⁺,K⁺, Pb²⁺, Ca²⁺, Sr²⁺, Bi³⁺, Y³⁺, Mn³⁺ and La³⁺, B represents one kind ortwo or more kinds of ions selected from the group consisting of Ru³⁺,Fe³⁺, Ti⁴⁺, Zr⁴⁺, Cu⁴⁺, Nb⁵⁺, Ta⁵⁺, V⁵⁺, W⁶⁺ and Mo⁶⁺, and m representsa natural number of 1 or more, LnBa₂Cu₃O₇, Z₂Ba₂Ca_(n−1)Cu_(n)O_(2n+4)or ZBa₂Ca_(n−1)Cu_(n)O_(2n+3), wherein Ln represents one kind or two ormore kinds of ions selected from the group consisting of Y, La, Ce, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, Z represents one kindor two or more kinds of ions selected from the group consisting of Bi,Tl and Hg, and n represents a natural number of from 1 to 5, and (ii) acatalytic complex oxide material of one or more kinds of oxides selectedfrom the group consisting of CaO, PbO, ZnO, SrO, MgO, FeO, Fe₂O₃, B₂O₃,Al₂O₃, In₂O₃, Y₂O₃, Sc₂O₃, Sb₂O₃, Cr₂O₃, Bi₂O₃, Ga₂O₃, CuO₂, MnO₂, ZrO₂,TiO₂, MoO₃, WO₃, V₂O₅ and a lantanoid oxide; and (iii) SiO₂.
 7. Asolution as claimed in claim 6, wherein the complex oxide is an oxiderepresented by formula X₂SiO₅ or X₄Si₃O₁₂ (wherein X represents Ca²⁺,Pb²⁺, Zn²⁺, Sr²⁺, Mg²⁺, Fe²⁺, Fe³⁺, B³⁺, Al³⁺, In³⁺, Y³⁺, Sc³⁺, Sb³⁺,Cr³⁺, Bi³⁺, Ga³⁺, Cu⁴⁺, Mn⁴⁺, Zr⁴⁺, Ti⁴⁺, Mo⁶⁺, V⁵⁺, La³⁺, Ce³⁺, Pr³⁺,Nd³⁺, Pm³⁺, Sm₃₊, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, orLu³⁺).
 8. An oxide material film coated substrate in which anelectroconductive material film and a film of an oxide material asclaimed in any one of claims 1 to 5 thereon are formed on a substrate.9. An element in which a lower electrode, a film of an oxide material asclaimed in any one of claims 1 to 5 and an upper electrode in this orderare formed on a substrate.
 10. A semiconductor element in which a filmof an oxide material film as claimed in any one of claims 1 to 5 and anelectroconductive material film are formed on a semiconductor substrate,and further a pair of impurity diffusion layers are located on bothsides of the preceding electroconductive material film and on thepreceding semiconductor substrate surface.